Is it possible to have two official repositories like "http://ftp.is.debian.org/debian/ unstable main" and "http://ftp.uk.debian.org/debian/ unstable main". If some packages in the "is" one don't work it will try the "uk" one?

The reason I'm asking is because I use http://ftp.is.debian.org/debian/ but lately I often get an error saying the package couldn't be gotten, even though it was on the list (and I had done apt-get update), then I would switch over to the "uk" one and use that to download the packages I couldn't get, I would like this to be automatic, so that if the main one fails to get some package it will try the other one..

I'm skeptic though that this is possible in some "default" way.